For mechanical structure components including transmission components for automobile and the like, which are used under stress continuously for a long time, fatigue strength and wear resistance are required. Accordingly, these mechanical structure components are usually manufactured by surface hardening heat treatment processing a steel material to a desired component shape followed by surface hardening heat treatment. As a steel surface becomes hard and compressive residual stress is introduced to a steel surface layer portion by performing the surface hardening heat treatment, the fatigue strength and the wear resistance of the component are improved.
Carburizing and nitriding are shown as the typical surface hardening heat treatment. The carburizing heats a steel to a temperature of an A3 transformation point or more so that carbon diffuses and penetrates (carburize) at the surface layer portion of the steel. Usually, a high-temperature steel after carburizing is directly quenched to achieve surface hardening of the steel. In this carburizing, since the carbon is diffused and penetrated at the steel surface layer portion in a high-temperature range of the A3 transformation point or more, the carbon diffuses and penetrates from the steel surface to a comparatively deep position. This allows obtaining a large hardened layer depth.
However, in the case where the carburizing is selected as the surface hardening heat treatment, deterioration in accuracy of component shape caused by transformation strain and heat strain during the quenching cannot be avoided. In a state where the steel remains to be as-quenched after the carburizing, toughness of the steel is considerably deteriorated. Accordingly, when manufacturing components through the carburizing, to achieve correction of a component shape and recovery of toughness, performing tempering (for example, press tempering treatment) is necessary after the quenching. This increases the number of manufacturing steps, extremely disadvantageous in terms of a production cost.
On the other hand, the nitriding heats a steel to a temperature of an A1 transformation point or less to diffuse and penetrate (nitride) nitrogen at the steel surface layer portion. This ensures surface hardening of the steel without quenching like the carburizing. That is, since the nitriding features a comparatively low treatment temperature and does not involve a phase transformation of the steel, manufacturing the components through the nitriding allows maintaining good accuracy of component shape. However, gas nitriding using ammonia gas requires considerably long nitriding time, approximately 25 to 150 hours, and therefore is not suitable to automotive parts and the like supposed to be mass produced.
Soft-nitriding has been recently popular as treatment for advantageously solving the problem observed in the gas nitriding. The soft-nitriding is nitriding to quickly progress a nitriding reaction using carburizing atmosphere. Although obtained steel surface hardness is lower than the conventional nitriding (gas nitriding), this soft-nitriding allows significant shortening of the nitriding time.
The soft-nitriding is broadly classified into a method of nitriding in salt bath and a method of nitriding in gas. The method of nitriding in salt bath (salt bath soft-nitriding) uses a cyanogen-based bath; therefore, measures to prevent environmental pollution is necessary. On the other hand, since the method of nitriding in gas (gas soft-nitriding) uses mixed gas with the main component of ammonia, this method emits less discharge causing the environmental pollution. Due to the above-described reasons, an adoption ratio of the gas soft-nitriding, which nitrides a steel in gas, has been particularly increased among the soft-nitriding.
On the other hand, conventionally, mechanical structure components such as transmission components for automobile are generally manufactured by machining an intermediate product obtained by casting and forging and then processing and joining the intermediate product to a desired shape. However, recently, steel sheets (thin steel sheets) have been actively used as a raw material. Performing press processing or the like on the steel sheet (thin steel sheet) shapes the steel sheet into a desired shape, thus manufacturing the component. This shortens the manufacturing processes than the conventional manufacturing processes, allowing significant reduction of the production cost. From this background, demands on the steel sheet for soft-nitriding excellent in formability, which is suitable as a material of the mechanical structure component including the transmission component for automobile or the like, have been increased, and accordingly, various techniques have been proposed up to the present.
For example, Patent Literature 1 and Patent Literature 2 disclose a method for manufacturing steel sheet for nitriding excellent in formability and the steel sheet for nitriding excellent in formability having a composition described below. A steel has a chemical composition containing, by weight ratio, C: 0.01 to less than 0.08%, Si: 0.005 to 1.00%, Mn: 0.010 to 3.00%, P: 0.001 to 0.150%, N: 0.0002 to 0.0100%, Cr: more than 0.15 to 5.00%, Al: more than 0.060 to 2.00%, and further containing one or two of Ti: 0.010% or more to less than 4C [%], and V: 0.010 to 1.00%. The steel is coiled at 500° C. or more after hot rolling, or is further cold-rolled at a rolling reduction of 50% or more after the coiling followed by recrystallization annealing. According to this technique, by controlling a C content, which adversely affects the formability, to less than 0.08% and by containing Cr, Al, or the like as a nitriding promoting element, it is described that steel sheet for nitriding excellent in formability and nitridation is obtained.
Patent Literature 3 proposes the following steel for soft-nitriding. The steel for soft-nitriding has a chemical composition containing, by mass %: C: 0.03% or more to less than 0.10%, Si: 0.005 to 0.10%, Mn: 0.1 to 1.0%, and Cr: 0.20 to 2.00% and as impurities, S: 0.01% or less, P: 0.020% or less, sol. Al: 0.10% or less, and N: 0.01% or less and the balance comprising Fe. The steel for soft-nitriding has a ferrite grain size of grain size number 5 or more to 12 or less specified by JIS G 0552. According to the technique, it is descried that since expensive element of Ti, V, or the like is not added, an inexpensive steel sheet can be obtained. Moreover, it is descried that refining a crystal grain diameter of the steel allows obtaining a steel sheet excellent in press processability.
Patent Literature 4 proposes the following thin steel sheet for nitriding. The thin steel sheet for nitriding has a chemical composition containing, by mass %: C: more than 0.01% to 0.09% or less, Si: 0.005 to 0.5%, Mn: 0.01 to 3.0%, Al: 0.005 to 2.0%, Cr: 0.50 to 4.0%, P: 0.10% or less, S: 0.01% or less, and N: 0.010% or less. Optionally, the thin steel sheet for nitriding further contains one or two or more selected from V: 0.01 to 1.0%, Ti: 0.01 to 1.0%, and Nb: 0.01 to 1.0%. A grain boundary area Sv per unit volume is set at 80 mm−1 or more to 1300 mm−1 or less. According to the technique, by containing a nitride forming element, Cr, Al, V, Ti, Nb, or the like in a range of not inhibiting the formability of the steel sheet as well as regulating the grain boundary area per unit volume in a predetermined range, it is described that both high surface hardness and sufficient hardening depth can be obtained after nitriding.
Patent Literature 5 proposes a steel sheet for soft-nitriding having a composition containing: C: 0.03 to 0.10 mass %, Si: 0.5 mass % or less, Mn: 0.1 to 0.6 mass %, P: 0.04 mass % or less, S: 0.04 mass % or less, Al: 0.005 to 0.08 mass %, Cr: 0.4 to 1.2 mass %, Nb: 0.002 mass % or more to less than 0.01 mass %, and N: 0.01 mass % or less. According to the technique, it is described that containing a trace of Nb allows obtaining a steel sheet for soft-nitriding featuring both processability and fatigue property.    Patent Literature 1: Japanese Unexamined Patent Application Publication No. 9-25513    Patent Literature 2: Japanese Unexamined Patent Application Publication No. 9-25543    Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2003-105489    Patent Literature 4: Japanese Unexamined Patent Application Publication No. 2003-277887    Patent Literature 5: Japanese Unexamined Patent Application Publication No. 2009-68057